In the United States, the supply of high grade fossil fuels (coal and oil) of low sulfur and of high heat content is approaching exhaustion, and power plant operators and other producers of energy are now turning to high sulfur fuels. However, in the absence of special precautions such fuels generally cannot be burned lawfully in urban areas because they release excessive quantities of sulfur dioxide. Smelters of sulfur-containing ores face similar restraints. National clean air standards limit sulfur dioxide emissions to low values to prevent excessive pollution of the atmosphere.
It is now possible to desulfurize flue gases adequately to meet these standards, but the processes presently used for this purpose are complex and costly; moreover, they generally do not recover the sulfur in economically useful form. Economic recovery of sulfur is an important consideration, because a moderately sized (million kilowatt) electrical power plant operating on coal of moderate (3%) sulfur content of moderate (12,500 B.T.U./lb.) fuel value releases about 480 tons of sulfur dioxide (equivalent to about 240 tons of sulfur) per day.
It has been proposed to desulfurize flue gases by an alkalized alumina process wherein a raw absorbent solid in the form of small spheres of dawsonite [NaAl(CO.sub.3)(OH).sub.2 ] is activated at 1,200.degree. F. to form a high-porosity, high surface area sodium alumina. The dawsonite reacts with the sulfur dioxide in the flue gas to form sulfates which are then converted to aluminates in the presence of a reducing gas at 1,200.degree. F. A process of this type is described in "Control Techniques for Sulfur Oxide Air Pollutants" (U.S. Dept. of Health, Education and Welfare, Washington, D.C., Jan. 1969); see also U.S. Pat. Nos. 3,551,093 and 3,580,702. The process has serious disadvantages in that it requires the formation and use of friable spheres which are difficult to manufacture and which have a comparatively short life, and in that it does not convert the sulfur dioxide gas to compact, inert, easily transportable and high-value form.